Micromanipulation studies of individual hemopoietic progenitors and paired progenitors derived from single cells have provided strong evidence for the stochastic process of lineage restriction of multipotential progenitors. This next year, part of our efforts will be directed toward clarification of the question of how progressive this restriction is. For this purpose, we plan to carry out serial replating of paired progenitor cells. The second major effort will be directed toward the question of the relationship between the hemopoietic system and the lymphopoietic system. There have been a large number of reports claiming the common origin of T- and B-lymphocytes and the hemopoietic system. Most of those reports are in vivo observations and have specific defects. There are a few reports on cell culture studies claiming the presence of T- and B-lymphocyte precursors in human and murine multilineage colonies. However, the evidence is only statistical and no direct proof of the clonal origin of the mixed colonies has yet been presented. We plan to grow multilineage colonies from single cells, expand the candidate T- and B-lymphocyte precursors in specific suspension or colony culture assays and test the presence of lymphocytes by use of immunofluorescent techniques.